Destruction of cadmium cyanide waste by alkaline chlorination treatment

Sludge characteristics and supernatant quality were studied in laboratory scale tests on alkaline chlorination treatment of cadmium cyanide solutions. Jar tests were conducted at pH values ranging from 7 to 11, utilizing two forms of hypochlorites as oxidants. In addition, synthetic solutions were prepared without cyanide to study the precipitation process independently of the oxidation step in alkaline chlorination. Jar tests were evaluated in the general areas of supernatant and filtrate quality, sludge settling and filterability characteristics, and precipitate mineral characteristics.

Results of the study indicate that at high initial cadmium concentrations calcium hypochlorite treatments yield sludges with better de watering characteristics than sodium hypochlorite treatments. However, sludge volume is greater in calcium hypochlorite treatments due to coprecipitation of calcium and cadmium carbonate sludges.

Distinctly different types of sludge settling behavior were found as a function of pH. In high pH solutions gelatinous sludges were typically obtained. For pH < 10, dense, grainy sludges were formed containing significantly less volume.

Projections from this work indicate that full-scale treatment at pH 9 would be advantageous. At this pH, cadmium levels are in the region of minimum solubility. In addition, dense, grainy sludges appear to be kinetically favored. Calcium hypochlorite treatment may offer advantages, but pH control problem.s and excessive sludge volumes must be handled.